With the development of single-chip integrated optical structures containing laser sources, passive and active waveguides and detectors, new materials are needed for making waveguides, "chip-to-chip" and "backplane interconnects" that can be patterned using processing technologies suitable for silicon and gallium arsenide electronic devices. Organic polymer films offer potential advantages over waveguides based on inorganic crystals, because the former can be processed at much lower temperatures; they are amenable to solution spin casting and other coating techniques; they have lower dielectric constants; and they can have large electro-optic or other nonlinear optical responses that are electronic in origin and therefore have low losses even in high frequency regimes.
Free-standing and embedded rib waveguides have been formed in organic films by generating refractive index patterns by methods such as (1) photochemical crosslinking, followed by dissolution of the remaining uncrosslinked material; (2) "photo-locking", i.e. photochemical attachment, dimerization or polymerization of a high refractive index monomer in a transparent polymer matrix film, followed by baking to remove the remaining volatile monomer from unirradiated areas; (3) patterned argon ion laser irradiation; and (4) thermal annealing. Further, photochemical bleaching of dye molecules in polymer matrices without crosslinking, dimerization or polymerization has been explored as a mechanism for changing the refractive index in organic thin films.
Formation of waveguide structures in nonlinear optical organic materials (including polymers) through the photochemical transformation of photoreactive functional groups (photodelineation) is disclosed in commonly assigned copending U.S. appl. Ser. No. 456,411, filed Dec. 26, 1989 by McFarland et al. for "Method for Forming Optically Active Waveguides". Also, Mohlman et al. [G. R. Mohlman et al., SPIE 1177 0-9, 67, Boston, September, 1989] have described the use of radiation from a mercury lamp to bleach films of a methacrylate polymer with a side chain 4-dimethylamino-4'-nitrostilbene photoreactive functional group to produce optical waveguide structures. Horn et al. have described similar laser and contact mask exposure patterning of channel waveguide structures in thin films of poly(methylmethacrylate) ("PMMA") containing dispersed monomeric nitrones [see, for example, Horn et al., "Polymeric Materials for Guided Wave Devices", The 1989 International Chemical Congress of Pacific Basin Societies, Honolulu, Hi., Dec. 17-22, 1989, Macr. 82]. Their monomeric nitrones were made by standard synthetic routes from the corresponding aldehydes and substituted hydroxylamines. Doses of only several mJ to hundreds of mJ per square centimeter were required to fully bleach thin films of nitrones in PMMA. The homopolymer poly(4-(N-phenyl-.alpha.-nitronyl)phenyl methacrylate) and copolymers of methyl methacrylate with 4-(N-phenyl-.alpha.-nitronyl)phenyl methacrylate and use thereof for laser direct writing of channel waveguides and passive integrated optical circuits are disclosed in commonly assigned co-pending U.S. appl. Ser. No. 664,248 now U.S. Pat. No. 5,176,983 filed 4 March 1991 by Horn et al. for "Polymeric Nitrones Having an Acrylic Backbone Chain."
Waveguides utilizing nitrones disclosed in the prior art are based on monomeric nitrones. We are not aware of any prior art disclosure of a synthetic route to polymers with nitrone functional groups. There are several reasons for this. First, the nitrones are excellent radical scavengers, as illustrated by the fact that monomeric nitrones are used as stabilizers for polymers especially during processing at elevated temperatures (e.g. melt processing). Thus, free radical polymerization of monomers containing nitrone functional groups is fully inhibited by the functional group itself. Second, attempted syntheses of nitrone polymers containing olefinic functional groups can result in dipolar cycloaddition. Apparently, there is no standard route to polymeric nitrones.
While photodelineation of organic films to form waveguides provides significant improvements over the prior art because it eliminates involved processing steps and avoids both thermal and etch damage to sensitive silicon and gallium arsenide electronic components, the existing materials have several shortcomings. In the known dye/polymer (guest/host) materials only low concentrations (&lt;30%) of the monomeric dye can be dissolved in the transparent host polymer matrices such as PMMA, polystyrene polyvinyl alcohol and polycarbonate. For example, concentrations of above about 9-13 percent of the azo dye disperse red #1 in PMMA result in phase separation and crystallization, producing a highly scattering film. Also, the photochemically reactive dye species is readily leached out of the host matrix by solvents used in subsequent processing steps, or it is baked out in the course of removal of residual spinning solvent, resulting in refractive index changes in the waveguide films. Further, the glass transition temperature of the guest/host film generally decreases with increasing dye molecule concentration, thus compromising the physical integrity of the film. In addition, all of the chromophores previously used in these systems, except for the nitrones, have low quantum yield photoreactions, and thus require high actinic radiation does. While the prior art polymeric systems avoid the problems of the guest/host systems--i.e. low concentration limit, bake-out and T.sub.g deterioration--the photochemical reaction quantum yields for their chromophores still tend to be low. The exposure doses required for refractive index changes (to 50% of saturation) generally are on the order of joules to hundreds of joules per square centimeter, with resultant exposure times of hours with standard lamps and laser sources. These high irradiation doses tend to degrade the polymer backbone, and result in cross linking or other adverse side reactions, with undesirable refractive index changes and deterioration of physical properties. Further, at these high irradiation levels, secondary photoproducts are generated which, even in trace amounts, can cause unacceptably large waveguide losses if they absorb in the visible or infrared regions.
There is a need for new polymeric materials whose refractive index can be changed by actinic radiation with low doses while avoiding or alleviating the above stated shortcomings of the prior art materials.